1. Field of the Invention
This invention relates to high-speed transmission of digital data over transmission channels of limited bandwidth. In particular, a transmission rate of three bits per cycle of bandwidth is attained in communication channels whose signal-to-noise ratio limits the number of transmitted levels that can be reliably distinguished in a multilevel channel signal.
2. Description of the Prior Art
In U.S. Pat. No. 3,388,330, issued to E. R. Kretzmer on June 11, 1968, the concept of communication channel shaping to effect controlled correlation between received signal samples is introduced. Such controlled signal shaping is called partial-response shaping because the impulse response to each signal input is so related to the signaling interval that the response within a signaling interval is only partial. The result is that intersymbol interference is allowed to occur, but it is structured in such a way that the binary significance of individual samples of the received signal is preserved. Symbol speeds at the maximum theoretical rate of two symbols per second per Hertz of bandwidth and the corresponding binary bit rate of two bits per second per Hertz are thus readily obtained in practical communication channels.
In my copending joint patent application with A. M. Gerrish, Ser. No. 639,870, filed May 19, 1967, Now U.S. Pat. No. 3,492,578 issued Jan. 27, 1970, it is further disclosed that by combining multilevel (more than two levels per symbol) signaling with partial-response encoding an equivalent binary signaling speed in excess of two bits per second per Hertz of channel bandwidth can be attained. Specifically, a speed log.sub.2 N bits per channel symbol is possible for N input levels per symbol. With the maximum partial-response symbol rate of two symbols per second per Hertz this gives a bit rate of 2 log.sub.2 N bits per second per Hertz.
Practically, N appeared to be restricted to powers of two so that an integral number m(m=log.sub.2 N) of binary input digits would be encoded on each level and so that there would be a direct correspondence between the N levels of the multilevel signal and the N possible combinations of the m binary digits. However, with partial-response encoding, the N baseband levels generate (2N-1) channel levels. Moreover, for each increase in the number of channel levels there is a signal-to-noise penalty that in many practical communication channels prohibits four-level baseband operation.
It is an object of this invention to adapt the partial-response principle to attain a speed capability for data transmission at rates of m bits per symbol, such that m is no longer restricted to being a positive integer, i.e., the binary signaling rate is a nonintegral multiple of the channel baud rate.
It is another object of this invention to increase the equivalent binary data transmission rate of a synchronous digital transmission system without changing the synchronous channel symbol rate itself.